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Pointers in C

Group Members:

Kavitha SreekumarNitin Chavan (GL)Sahitya GollapalliVinay Mandge

© 2010 Tieto Corporation

Contents

• Introduction to Pointer

• Pointer advantages

• Declaration of Pointers

• Pointer Arithmetic

• Pointers to Arrays and strings

• Pointers and Structures

• Pointers to functions

• Some issues with pointers

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Introduction to Pointer ?

• Derived data type

• “A pointer is a variable, which contains the address of

another variable.”

• In other words:

- A variable that holds Memory Address rather than its

content.

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Memory Cell

0

1

.

.

.

2

Quantity Variable

150 Value

4500 Address

Representation of variable

Memory OrganisationAddress

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Pointer advantages

• To point to different data structures.• To achieve clarity and simplicity.• More compact and efficient coding.• To return multiple values via functions.• Dynamic memory allocation.

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• Variable name must be preceded by Asterisk (*)

• Should have some data type

• E.g.:-

• Declaration :-

int *ptr; // indicates ptr is pointer to an integer variable.

• Initialize :-

ptr= &x; // ptr is pointing to x.

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Declaration of Pointers

Use of & and *• • When is & used?

• • When is * used?

• • & -- "address operator" which gives or producesthe memory address of a data variable.

• * -- "dereferencing operator" which provides thecontents in the memory location specified by aPointer.

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Pointer Arithmetic

• Operations such as Increment & Decrement.

• Ptr++ ; // Points to next memory location.

• E.g.:-

# include <stdio.h>

char movie[] =“Jurassic park”;

main()

{

char *ptr;

ptr=movie;

printf(“%s”,movie);

printf(“%s”,ptr);

ptr++;

printf(“%s”,movie);

printf(“%s”,ptr);

}

• Declaration :

int x[5] = { 1, 2, 3, 4, 5};

int *nptr ;

nptr = &x[0]; // Points to address location 1000.

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Pointer and Array

X[0] X[1] X[2] X[3] X[4]

1 2 3 4 5

1000 1002 1004 1006 1008

Elements

Values

Address

Base address

Pointers to Array • Declaration : int *nptr ; nptr = number[0] ; or nptr = number ;

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1 2 3 4 5

1000 1002 1004 1006 1008

X[0] X[1] X[2] X[3] X[4]index

value

address

nptr

Array of Pointers (Ragged arrays)• Declaration : char *name[3] = { “New Zealand” , “Australia” , “India” , }

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name[0]

name[1]

name[2]

New Zealand

Australia

India

Pointers to String

• Declaration :

char *str = “good” ;

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G O O D \0

str

Example : char *cptr = name ;

int length ;

while (*cptr != ‘\0’) ;

length = cptr – name;

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Pointers to functions

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Pointers and Functions

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• Pointers can be used to pass addresses of variables to called functions, thus allowing the called function to alter the values stored there.This is known as "call by reference" since we are referencing the variables.

• As shown swap function modified from a "call by value" to a "call by reference". Note that the values are now actually swapped when the control is returned to main function.

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Pointers with Functions (example)

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#include <stdio.h>void swap ( int a, int b ) ;int main ( ){ int a = 5, b = 6; printf("a=%d b=%d\n",a,b) ; swap (a, b) ; printf("a=%d b=%d\n",a,b) ; return 0 ;}

void swap( int a, int b ){ int temp; temp= a; a= b; b = temp ; printf ("a=%d b=%d\n", a, b);}Results:

a=5 b=6a=6 b=5a=5 b=6

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Pointer to a function

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• Pointer to function are the pointers which have the ability to call the function pointed to by them. As the ordinary pointers take the address of variable and can change the value pointed by them, the function pointer almost does the same.

• example• #include <stdio.h>

#include <conio.h>int mul(int a, int b, int c) {  return a*b*c;}void main() {  int (*function_pointer)(int, int, int);  function_pointer = mul;  printf("The product of three numbers is:%d", function_pointer(2, 3, 4));  getch();}

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Pointers and structures

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Pointers to Structures • We declare pointers to structures the same way we declare

any other pointers: by preceding the variable name with an asterisk in the declaration. We could declare two pointers to struct complex with

• struct complex *p1, *p2; And, as before, we could set these pointers to point to actual variables of type complex:

• p1 = &c2; p2 = &c3; Then, • *p1 = *p2 would copy the structure pointed to by p2 to

the structure pointed to by p1 (i.e. c3 to c2), and • p1 = p2would set p1 to point wherever p2 points.

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Some issues with pointers

advantages and disadvantages

• Pros efficiency convenience• Cons error prone difficult to debug

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References• Programming in ANSI c by E Balaguruswamy.

• need pointers in c ebook by yashwant kanetkar.

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Thank You